Achromatic optical elements are essential in commercial and medical therapeutic irradiation systems because the primary attribute for such operations is the relatively high beam intensity and control thereof. A typical high beam current accelerator, such as the microwave linear accelerator, achieves the required beam intensities but the energy distribution is rather wide. In order to utilize the available beam it is therefore necessary to introduce optical lements which are relatively insensitive to the energy distribution of the beam. In particular it is desirable for x-ray apparatus to concentrate an intense beam onto a small beam spot on the x-ray target to obtain an x-ray source sufficiently small in relationship to the targeted irradiation region.
Beam deflection systems in commercial irradiation and medical therapy applications are ordinarily subject to mechanical and geometrical constraints incident to the maneuverability of the apparatus, shielding and collimation of irradiation flux and as well as economic considerations in the construction of such apparatus.
One achromatic beam deflection system of the prior art is described in U.S. Pat. No. 3,867,635 commonly assigned with the present invention. In this apparatus the beam traverses three uniform field sector magnets and two intermediate drift spaces, undergoing a 270.degree. deflection for incidence upon the x-ray target. The sector magnet poles are precisely specified in regard to the sector angles. The angles of incidence and egress of the beam with respect to each sector and a shunt of complex shape occupies the intermediate spaces as well as the entrance and exit regions of the deflector to assure required field free drift spaces. The mutual internal alignment of all components of the deflector is essential to achieve the performance of this prior art device as well as is the alignment of the assembled deflector with the accelerator beam.
Another prior art system is known from U.S. Pat. No. 3,379,911 wherein 270.degree. deflection is accomplished in a uniform field to which there is introduced in the vicinity of the deflection midpoint (135.degree.) a gradient region, such that the magnetic field in this gradient region increases radially in the plane of deflection toward the outer portion of accepted trajectories. Thus, those trajectories characterized by a large radius of curvature (in the absence of a gradient) are subject to a somewhat more intense field than would be the trajectories for smaller radii of curvature. Proper adjustment of the gradient shim yields first order achromatic deflection through the desired angle.
It is desirable in all of the described systems for the deflector to introduce no substantial momentum dispersion of the beam and to produce at the exit plane a faithful reproduction of conditions encountered at the entrance plane of the system.